Insulin resistance contributes to the pathogenesis of several disease states including obesity and noninsulin-dependent diabetes mellitus (NIDDM). We have investigated the nature of the genetic factors that predipose to the development of insulin resistance. Mutations have been identified in the insulin receptor gene in patients with several genetic syndromes associated with insulin resistance. Some mutations cause insulin resistance by decreasing the number of receptors on the cell surface by one of several mechanisms; other mutations cause insulin resistance by impairing one or more functions of the insulin receptor. We have developed an animal model of this disease: a mouse in which the insulin receptor gene has been inactivated by homologous recombination. Mice lacking insulin receptors were normal in size at birth, but developed severe diabetic ketoacidosis causing death within the first 2-3 days post-natally. In contrast, mice who were heterozygous for a null allele of the insulin receptor gene had slightly elevated levels of plasma insulin, but normal glucose levels. Breeding studies were carried out to obtain mice that were "double heterozygotes", i.e., heterozygous for null alleles of the genes encoding the insulin receptor (IR) and insulin receptor substrate-1 (IRS-1). In mice that were heterozygous for inactivating mutations in both genes, the two mutations worked synergistically to cause insulin resistance. In separate studies, we investigated the significance of naturally occurring variants of the amino acid sequence of the human gene encoding IRS-1. Previously, we and others had identified several variant sequences that appear to be more prevalent among patients with noninsulin-dependent diabetes mellitus. We have expressed several of the "mutant" proteins, with the major emphasis upon the most common variant: the Gly972Arg mutant. However, the fact that we did not detect a functional defect in recombinant Arg-972 mutant IRS-1 raises questions about the hypothesis that this amino acid substitution causes insulin resistance, thereby predisposing to NIDDM.